The present invention relates to transmissive type photodetector tubes and more particularly to transmissive type photodetector tubes designed to be non-responsive to Cerenkov radiation generated in the body of the tube.
A transmissive type photodetector tube is a tube with a photocathode of a material which emits electrons in response to impinging photons with the electrons being emitted from a surface opposite the surface on which photons impinge. Heretofore, some transmissive type photodetector tubes have comprised an evacuated envelope with a transparent face plate. The photocathode is on the face plate within the evacuated envelope. In contrast, a reflective type photodetector tube is a tube with a photocathode of a material which emits electrons in response to impinging photons with the electrons being emitted from the same surface as that on which photons impinge. Some of the reflector type photodetector tubes have comprised an evacuated envelope with a transparent face plate. The photocathode is mounted in the envelope spaced apart from the face plate, since the photoelectrons are emitted substantially in the direction opposite to the direction of the impinging photons, i.e. the photoelectrons are directed toward the source of the photons or toward the face plate. Thus, the gap between the reflective photocathode and the face plate is fairly large, to allow room for electrons emitted from the photocathode. Typically this gap is on the order of an inch or more. However, in certain applications it is desirable to have the photocathode as close as possible to the source of the photons. By having the photocathode close to the source of photons, the photocathode can collect more of them because there is less divergence in the beam of photons. In these uses a transmissive type photodetector is highly desirable because it is able to get closer to the source of photons than a reflective type photodetector.
One such application is in high energy physics, such as experiments involving an accelerator. However, in such application high speed charged particles are often found and Cerenkov radiation results. Cerenkov radiation is the emission of light by a high speed charged particle when the particle passes through a transparent, non-conducting material, such as the face plate of a photodetector tube, at a speed greater than the speed of light in the material. In those applications where there are high speed charged particles sufficient to cause Cerenkov radiation, a transmissive type photodetector tube with the photocathode on the face plate in the tube would detect not only the light that it is suppose to detect, but the photodetector tube would also respond to the light caused by the high speed charged particles passing through the face plate of the photodetector tube. Thus, extraneous light results and the recordation of the extraneous light occurs.